1. Field of the Invention
The present invention relates to a vertical type heat-treating apparatus and a heat-treating method for effecting heat treatment such as an oxidation or diffusion process on an object to be treated or a target object, such as a semiconductor wafer.
2. Description of the Related Art
In effecting a heat treatment such as an oxidation or diffusion process on a semiconductor wafer, a plurality of wafers are mounted on a quartz glass boat and this boat is carried inside a process tube of a heat-treating apparatus. Conventionally, horizontal type heat-treating apparatuses which have a process tube disposed horizontal along its length, have been used as such a heat-treating apparatus for wafers (refer to U.S. Pat. No. 3,828,722).
However, recent enlargement of wafers and an increased number of wafers to be mounted on a boat (e.g., 150 wafers) inevitably enlarges the heat-treating apparatuses, so that the following shortcomings arise for the horizontal type. Since the process tube is disposed horizontal along its length in the horizontal type heat-treating apparatus, a large mounting area should be provided according to the length. Therefore, enlargement of the apparatus inevitably increases the mounting area, which makes the apparatus inappropriate for installment in a clean room whose cost per unit area is high. Further, since the tare of a boat having wafers mounted thereon is directly applied to the process tube in the horizontal type heat-treating apparatus, the process tube is unlikely to be able to endure the total weight of the boat increased by an increase in the number of wafers. In addition, since the boat carrying-in and carrying-out directions differ from the direction in which the tare of the boat is applied, the boat is more likely to directly contact the process tube due to bending of a carrying-in fork, resulting in easy generation of particles.
To overcome the above problems, vertical type heat-treating apparatuses have recently been used for heat treatment of semiconductor wafers (refer to Japanese Utility Model Disclosure No. 61-183525 and Japanese Patent Disclosure No. 58-60552). In the vertical type heat-treating apparatuses, the process tube is disposed vertically, a boat having semiconductor wafers mounted thereon is carried in the process tube through the opening at its lower end, the interior of the process tube is heated by a heater disposed around the tube, and a reaction gas is introduced in the tube to subject the wafers to heat treatment.
Since the vertical type heat-treating apparatuses can have a smaller mounting area than the horizontal type and can permit a boat to be disposed in the process tube without contacting it, the aforementioned shortcomings of the horizontal type heat-treating apparatuses can be overcome. In addition, the vertical type heat-treating apparatuses have higher uniformity of a temperature applied to the wafers and higher uniformity of a gas distribution, as compared with the horizontal type, thus ensuring higher yield. Therefore, the vertical type heat-treating apparatuses can cope with an increase in the diameter of the process tube resulting from enlargement of wafers and can be easily automated.
To apply such a vertical type heat-treating apparatus to, for example, an apparatus for providing a silicon epitaxial growth, the interior of the process tube is maintained at about 1000.degree. C. in an epitaxial growth process. If the wafer is taken out from the process tube from the high-temperature environment of about 1000.degree. C. immediately after the completion of this process, an undesired oxidization film may be formed on the wafer surface or the grown epitaxial film may be adversely affected. It is therefore necessary to cool the wafer to about 800.degree. C. before it is removed; this may be done by reducing the output of the heater around the process tube or turning off the heater power to cool the wafer to about 800.degree. C. within the process tube.
Since, as mentioned earlier, the vertical type heat-treating apparatuses are used to cope with large numbers of wafers, however, the heater generally has a large heat capacity so that the heater itself is not quickly cooled even by turning off the heater power. Accordingly, it takes a long time to reduce the temperature of wafers in the process tube to 800.degree. C. from about 1000.degree. C., several times longer than the time required for the epitaxial grown. Even if the epitaxial grown process itself is completed in, for example, about 10 minutes, therefore, the time required for the entire heat treatment for one cycle becomes longer than one hour, so that the throughput cannot be improved.